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194 Modern Analytical Chemistry

depth. Grab samples are conveniently collected by submerging a capped bottle
below the surface and removing the cap. The air–water interface, which may be en-
9
riched with heavy metals or contaminated with oil, is avoided when collecting the
sample. After the sample bottle is filled, the cap is replaced and the bottle removed.
Slowly moving streams and rivers, lakes deeper than 5 m, estuaries, and oceans may
show substantial stratification. Grab samples from near the surface can be collected
as described earlier, whereas samples at greater depths are collected with a weighted
sample bottle that is lowered to the desired depth. Once it has reached the desired
depth, the sample bottle is opened, allowed to fill, and closed before retrieving.
Grab samples can be analyzed individually, giving information about changes in the
analyte’s concentration with depth. Alternatively, the grab samples may be pooled
to form a composite sample.
Wells used for collecting groundwater samples must be purged before the sam-
ple is collected, since the chemical composition of water in the well-casing and in
the adjacent matrix may be significantly different from that of the surrounding
groundwater. These differences may result from contaminants introduced when
drilling the well, or differences in the groundwater’s redox potential when exposed
to atmospheric oxygen. In general, wells are purged by pumping out a volume of
water equivalent to several well-casing volumes, or until the water’s temperature,
pH, or specific conductance are constant. Samples collected from municipal water
supplies must also be purged since the chemical composition of water left standing
in pipes may differ significantly from the treated water supply. Samples are collected
at faucets after flushing the pipes for 2–3 min.
Samples from municipal wastewater treatment plants and samples of industrial
discharges often are collected as 24-h composites. Samples are obtained using an
automatic sampler that periodically removes individual grab samples. The volume
of each sample increment and the frequency of sampling may be constant or may
vary in response to changes in flow rate.
Sample containers for collecting solutions are made from glass or plastic. Con-
tainers made from Kimax or Pyrex brand borosilicate glass have the advantage of
being sterilizable, easy to clean, and inert to all solutions except those that are
strongly alkaline. The disadvantages of glass containers are cost, weight, and the
likelihood of breakage. Plastic containers are made from a variety of polymers, in-
cluding polyethylene, polypropylene, polycarbonate, polyvinyl chloride, and Teflon
(polytetrafluoroethylene). Plastic containers are lightweight, durable, and, except
for those manufactured from Teflon, inexpensive. In most cases glass or plastic bot-
tles may be used, although polyethylene bottles are generally preferred because of
their lower cost. Glass containers are always used when collecting samples for the
analysis of pesticides, oil and grease, and organics because these species often inter-
act with plastic surfaces. Since glass surfaces easily adsorb metal ions, plastic bottles
are preferred when collecting samples for the analysis of trace metals.
In most cases the sample bottle has a wide mouth, making it easy to fill and re-
move the sample. A narrow-mouth sample bottle is used when exposing the sample
to the container cap or to the outside environment is undesirable. Unless exposure
to plastic is a problem, caps for sample bottles are manufactured from polyethylene.
When polyethylene must be avoided, the container cap includes an inert interior
liner of neoprene or Teflon.

Sample Preservation Once removed from its target population, a liquid sample’s
chemical composition may change as a result of chemical, biological, or physical
processes. Following its collection, samples are preserved by controlling the solu-